1. Field of the Invention
The present invention relates to a system and a method for three dimensional measurement wherein a detection light such as a slit light is cast onto an object to measure a shape of the object.
2. Description of Prior Art
A non-contact type three dimensional measurement apparatus (three dimensional camera) called as a range finder can measure a shape of a matter at a faster speed than a contact type apparatus. Therefore, it is used as an apparatus for inputting an image for a computer graphic (CD) system or a computer aided design (CAD) system, measurement of a material, optical recognition of a robot and the like.
Slit light projection is known as a measurement technique suitable for a range finder, wherein an object is scanned optically and a distance image (three dimensional image) is calculated by using the principle of trigonometry. It is one of active measurement techniques which cast a detection light to pickup an image of an object. A distance image is a set of pixels representing three dimensional positions at a plurality of positions on the object. An operation to determine a distance image from pickup information is performed by a range finder itself or by an external information processor such as a computer system. In slit light projection, a linear slit light is used as the detection light, but other techniques using a spot light, a step light, a density pattern light or the like are also known.
In general, measurement information obtained by a range finder is sent to an on-line information processor or through a storage medium to an off-line information processor. Then, it is subjected to predetermined processing such as analysis, edition or storage of the information.
In a prior art range finder, one measurement is performed in response to a command received from the external or to a switch operation by a user. However, if a range finder performs measurements periodically, a range of uses of three dimensional measurement will be expanded. For example, it can be used more practically for inspecting a shape of an object in a production line in a factory, for optical recognition of a moving robot, for a watch system for security guard, or the like. When distance images obtained by periodic three dimensional measurements in a time series are compared, a move of an object back and forth or a change in shape can be recognized easily.
However, prior art range finders have problems on repeated pickup. For example, in a system where an external information processor gives a start command to a range finder, the external information processor has to send a start command for each of periodic measurements. Then, communication sequence becomes complicated. Further, in a conventional three dimensional measurement system, except in a special system used only for displaying a distance image, an information processor performs data processing such as data analysis and storage. Then, if measurements are performed periodically, the information processor has to recognize that the latest measurement information is sent from the range finder and has to store it for data processing.
A prior art portable range finder which can selects a pickup distance has a sensor for measuring a distance to an object. The measured distance is used for autofocusing and for setting a range of pickup angle. The distance measurement is performed before optical scanning for each pickup (or measurement).
In the range finder, active distance measurement can be performed precisely with use of an optical system for the measurement. Because it has a scanner, it is easy to expand a viewing angle for distance measurement by projecting a light in a plurality of directions. However, if active distance measurement is compared with passive distance measurement, at distance range which can be measured is narrower for the active distance measurement. Then, it is not sure a projection in a direction can catch the object. Then, when the direction of the projection has to be changed, the measurement time becomes longer.
Because the prior art range finder performs the distance measurement for each measurement, the measurement time by using the above-mentioned precise optical measurement is long, especially for the repeated pickups.
In a prior art range finder, the movement of the lens has to be forbidden during a pickup in order to prevent a change in the pickup conditions. Then, if a distance to an object is changed largely during a measurement, for example, if the object moves largely during a measurement, the distance measurement cannot follow the movement of the object, and a distance measurement controller loses sight of the object. If the object is lost, the measurement conditions such as focusing and light intensity cannot be set correctly, and the reliability of the measurement is worsened. Further, if the measurement is performed in a defocused state, the precision of the obtained distance image is lowered. Further, if the object is lost, the distance measurement may take a longer time to catch the object.